Bytesource wrote:
I am currently reading "Programming Clojure" but got stuck at the
destructuring done in the "head-overlaps-body?" function call that is
part of the "snake" game:
(defn head-overlaps-body? [{[head & body] :body}]
(includes? body head))
;; page 200 of the pdf version
I can not figure out what {[head & body] :body} actually means here.
Some simple examples might help -
(def snake {:body [[1 2] [2 3] [4 5] [6 4]]})
So snake is a map which has the key :body whose value is a vector of
vectors.
user> (def body-vals (:body snake))
[[1 2] [2 3] [4 5] [6 4]]
The first vector in body-vals is the actual head and the rest is the body.
user> (def head (first body-vals)
[1 2]
user> (def body (rest body-vals)
[2 3] [4 5] [6 4]
Now that we have separated out the data, we can proceed and use them in
our functions.
But then, that's a bit too much of code, since we can "destructure" the
snake very easily in the function args part like this -
[{[head & body] :body}]
What it does is that it first extracts the value of the :body key in the
map which is passed to the function.
Since the value of :body is supposed to be a vector, we can destructure
it further into head and body.
You can do the same kind of destructuring in a let form too.
I hope I was able to explain it to you. I agree, the examples were
pretty lame :)
Regards,
BG
--
Baishampayan Ghose <b.gh...@ocricket.com>
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